Various systems exist for the purpose of positioning one or more material layers (e.g., substrates, masks, etc.) in one or more locations to allow operations to be performed on the material layers. Some systems, such as alignment systems, attempt to position substrates by aligning one or more patterns on the material layers with the goal of a zero length offset.
For example, in one scenario, the material layers are positioned substantially parallel to each other and are spaced apart by a distance that is relatively short compared to the overall dimensions of the material layers. An imaging system having an optical axis disposed perpendicular to the plane in which the material layers reside is used to image one or more alignment markers on or within each layer. Relative position is established according to alignment marker positions. Alignment markers can be provided explicitly for alignment purposes, or can comprise landmarks intrinsically present on the material layer.
In addition to using more sophisticated microscopes to identify alignment markers, Moiré or other complex patterns may be used as alignment markers for the purpose of increasing resolution. However, the use of the sophisticated microscopes and the complex patterns may add prohibitive costs to the manufacturing process.
Further, with existing alignment systems, difficulties arise when the alignment markers overlap or are otherwise co-located in such a way that it is difficult or impossible to image the alignment markers individually. Alignment becomes even more difficult when the alignment markers are of different patterns. Specifically, the overlapping alignment markers may produce multiple peaks in an array of cross correlation values when performing cross-correlation of the two alignment markers. As such, due to the multiple peaks in the array, the location of the individual alignment markers cannot be positively determined. Also, the positioning of the material layer may be poorly quantized and may not be useful in instances where a non-zero offset is desired. Further, due to process variations, alignment systems that compare different patterns across different material layers may run into performance limitations.